This invention relates to an apparatus for drying a gaseous fluid under pressure, such apparatus as can be particularly utilized in fluid-pressure-operated systems for a vehicle having operating subsystems, such as a wheel-suspension system whose level changes as a function of the wheel load, or any other subsystem in which the consuming device is operated under variable volumes or a variable pressure.
One type of air-drying apparatus known in the art is described in the German Pat. No. DE 33 11 682, specifically in FIG. 2, which illustrates the air-drying apparatus having a switching arrangement with an activation piston which, when activated by a control fluid pressure, opens a discharge valve, and thereby initiates a regeneration phase. When the control fluid pressure is removed, the activation piston is retracted by a return spring, whereupon the discharge valve is closed and the normal operating phase is reestablished. The disadvantage with such an arrangement is that the control pressure must be separately-provided in order to switch from the normal operating phase to the regeneration phase. In the alternative, if the control fluid pressure were taken from the consuming device downstream of the air-drying apparatus, there must still be an external-line connection parallel to the air-drying apparatus, which not only increases installation costs, but also serves as a source of potential failure and maintenance problems.
Additionally, known air-drying apparatus typically operate at two distinct phases--the normal operating phase and the regenerative phase. In the normal operating phase, fluid pressure from a fluid pressure supply flows through the air-drying apparatus where the moisture in that fluid pressure is removed, to the fluid pressure consuming device which, in this example, is a vehicle having several fluid-pressure-operated subsystems including: the wheel-suspension system, the door-operating system, and the power-assist transmission. In the regeneration phase, fluid pressure from the fluid pressure supply is interrupted (either accidentally or intentionally), an outlet or exhaust valve is closed, and a fluid pressure supplied to the fluid pressure consuming device flows back through the air-drying apparatus and is re-dryed.
It would be a distinct advantage if a third phase of operation were available, such third phase corresponding to a standby phase, where the fluid pressure flow is naturally interrupted and the fluid pressure level at the input of the air-drying apparatus is nevertheless maintained. With the capability of the air-drying apparatus to operate at three distinct operating phases, the fluid pressure supply to the wheel-suspension system, for instance, allows for the wheel-suspension system to be filled during the normal operating phase, to be lowered or reduced in the regeneration phase, and to be maintained at the desired height in the standby phase.
Other known air-drying apparatus, which utilize an activation piston for operating a discharge valve, have further included a return spring for the replacement of the activation piston to its original position. Not only is such feature costly in terms of additional elements and machining of spring seats, but such feature is also prone to wear problems, such as fatigue of the spring, and to problems of alignment to keep the activation piston moving in the proper direction.
It is a further requirement for the fluid pressure system in which the air-drying apparatus operates to (in the regeneration phase) prevent the fluid pressure level in the consuming device from falling below a predetermined safety level. One method for controlling the reduction of fluid pressure in the consuming device (a method known in the prior art) is to place a throttling arrangement between the output device and the air dryer. This approach, however, has the disadvantage that another external means must be provided to interrupt the flow of fluid pressure out of the consuming device, since the air-drying apparatus merely limits such flow.